We have measured nonresonant and resonant Raman-scattering spectra from ZnO nanocrystals with an average diameter of 20 nm. Based on our experimental data and comparison with the recently developed theory, we show that the observed shifts of the polar optical-phonon peaks in the resonant Raman spectra are not related to the spatial phonon confinement. The very weak dispersion of the polar optical phonons in ZnO nanocrystals does not lead to any noticeable redshift of the phonon peaks for 20-nm nanocrystals. The observed phonon shifts have been attributed to the local heating effects. We have demonstrated that even the low-power ultraviolet laser excitation, required for the resonant Raman spectroscopy, can lead to the strong local heating of ZnO nanocrystals. The latter causes significant ͑up to 14 cm −1 ͒ redshift of the optical-phonon peaks compared to their position in bulk crystals. Nonresonant Raman excitation does not produce noticeable local heating. The obtained results can be used for identification of the phonons in the Raman spectra of ZnO nanostructures.

The authors report the thermal conductivity ͑K͒ of a variety of carbon films ranging from polymeric hydrogenated amorphous carbons ͑a-C:H͒ to tetrahedral amorphous carbon ͑ta-C͒. The measurements are performed using the 3 method. They show that thermal conduction is governed by the amount and structural disorder of the sp 3 phase. If the sp 3 phase is amorphous, K scales linearly with the CC sp 3 content, density, and elastic constants. Polymeric and graphitic films have the lowest K ͑0.2-0.3 W / mK͒, hydrogenated ta-C:H has K ϳ 1 W / mK, and ta-C has the highest K ͑3.5 W / mK͒. If the sp 3 phase orders, even in small grains such as in micro-or nanodiamond, a strong K increase occurs for a given density, Young's modulus, and sp 3 content.

We investigate the thermal conductivity of ultrathin tetrahedral amorphous carbon (ta-C) films on silicon, down to subnanometer thickness. For films with an initial sp3 content of 60%, the thermal conductivity reduces from 1.42to0.09W∕mK near room temperature as the thickness decreases from 18.5to∼1nm. The variation in ta-C film thickness is accompanied by changes in Young’s modulus, density, and sp3 content. The thermal resistance of the finite-thickness interface layer, which forms between ta-C and silicon, is ∼10−8m2K∕W near room temperature, thus producing a noticeable effect on thermal transport in ultrathin ta-C films.

Nanocrystalline diamond as an electronic material: An impedance spectroscopic and Hall effect measurement study

Recently proposed thermoelectric applications of quantum dot superlattices made of different material systems depend crucially on the values of the electrical and thermal conductivities in these nanostructures. We report results of the measurements of Hall mobility and thermal conductivity in a set of Ge 0.5 Si 0.5 /Si quantum dot superlattices. The average measured in-plane Hall mobility for the undoped Ge/Si quantum dot superlattices on a p-type substrate is 233.5 cm 2 V −1 s −1 at room temperature and 6.80 ϫ 10 3 cm 2 V −1 s −1 at 77 K. The average value of the thermal conductivity measured by 3 method is about 10 W/mK at room temperature and 3.5 W/mK at 77 K. In the low-temperature region, the thermal conductivity is proportional to T 0.7 − T 0.9 . Relatively large values of the carrier mobility and its temperature dependence suggest that the carrier transport in the investigated structures is likely of the band conduction type rather than hopping type. The thermal conductivity of the Ge 0.5 Si 0.5 /Si quantum dot superlattices is strongly reduced and has its peak value shifted toward the high temperatures as compared to the constituent bulk materials. Obtained results can be used for Ge x Si 1−x /Si quantum dot superlattice structure optimization for the high-temperature thermoelectric applications.